Self-supporting pigment layers for electrochromic display

ABSTRACT

An electrochromic data display and imaging device which may be formed by sandwich arrangement of the imaging area and the counter-electrode area, with a suitable self-supporting ion-conductive layer between.

BACKGROUND OF THE INVENTION

This invention relates to electro-optical devices whose electromagneticradiation absorption characteristics can be selectively altered byinfluence of a suitably controlled electric field. More particularly,this invention is directed to a sandwich type cell in which two layersof electrochromic material are separated by a self-supporting ionconducting medium.

In commonly assigned, copending U.S. applications, Ser. No. 41,153.Iadd.now abandoned, .Iaddend.Ser. No. 41,154 .Iadd.now abandoned.Iaddend.and Ser. No. 41,155 .Iadd.now U.S. Pat. 3,708,220, .Iaddend.allfiled May 25, 1970, and U.S. Pat. Nos. 3,521,941 and 3,578,843.[.; Ser.No. 41,153, abandoned and refiled as Ser. No. 211,857, Dec. 23, 1971,abandoned and refiled as Ser. No. 361,760, May 18, 1973, now copending;Ser. No. 41,154, abandoned and refiled, now pending; Ser. No. 41,155,now U.S. Pat. No. 3,708,220;.]..Iadd., .Iaddend.there are describedelectro-optical devices exhibiting a phenomenon known as persistentelectrochromism wherein electromagnetic radiation absorptioncharacteristic of a persistent electrochromic (EC) material is alteredunder the influence of an electric field. Such devices were employed insandwich arrangement between two electrodes. Coloration was induced bycharging the electrochromic film negative with respect to thecounter-electrode, employing an external potential. Thecounter-electrode can be the same as the persistent electrochromicmaterial or different.

By reversing the original polarity of the field or by applying a newfield, it was also possible to cancel, erase or bleach the visiblecoloration.

These steps of color induction and erasure are defined as cycling.

The devices described in the prior applications are effective to changetheir electromagnetic radiation transmitting properties under theinfluence of an electric field, and have extremely good visibility overa wide range of lighting conditions, including high ambient light.However, these EC displays normally feature a light color backgroundwhich consists of a thin layer of pigment mixed with a liquidelectrolyte to form an ion conducting layer. This layer serves to hidethe black counter-electrode and to provide good contrast with theblue-black EC film. It has been found that EC displays when stored forlong periods of time, especially when stored on edge, develop a problemof separation of the pigment background. This appears as a black crackin the light background due to the carbon counter-electrode showingthrough. Efforts to cope with this problem by providing thickened,gel-like electrolyte pastes resulted in slowing the switching speed asthe thickened electrolyte reduced the mobility of ions between theelectrodes.

It is, therefore, an object of this invention to provide anion-conducting medium having a color pigment incorporated which willretain a homogeneous consistency under varying physical conditions, overa long period of time.

This and other objects of the invention will become apparent as thedescription thereof proceeds.

SUMMARY OF THE INVENTION

The image display device is formed in a sandwich arrangement of anelectrochromic layer as an imaging area and a counter-electrode with aspacing of an ion-conducting medium, e.g. an electrolyte, between theareas. Means are provided for supplying electric current to thecounter-electrode layer. Any conventional means is suitable. Aparticularly advantageous means for electrical connection is to depositthe electrochromic imaging layer and counter-electrode on a conductivesurface, such as NESA glass. It is particularly advantageous toincorporate a pigment material with the electrolyte for greater contrastand for masking the counter-electrode.

The present invention discloses methods of preventing the pigment fromseparating in one area and compacting in another by binding the pigmentin a self-supporting structure. Another advantage provided is that theself-supporting pigment structure acts as a separator preventingelectrical shorting of the EC and counter-electrodes. The methods usedto prepare the pigment layer result in porous structures which do notrestrict ion flow and thereby maintain good switching speed.

DETAILED DESCRIPTION OF INVENTION

As used herein, a "persistent electrochromic material" is defined as amaterial responsive to the application of an electric field of a givenpolarity to change from a first presistent state in which it isessentially non-absorptive of electromagnetic radiation in a givenwavelength region, to a second persistent state in which it isabsorptive of electromagnetic radiation in the given wavelength region,and once in said second state, is responsive to the application of anelectric field of the opposite polarity to return to its first state.Certain of such materials can also be responsive to a short circuitingcondition, in the absence of an electric field so as to return to theinitial state.

By "persistent" is meant the ability of the material to remain in theabsorptive state to which it is changed, after removal of the electricfield, as distinguished from a substantially instantaneous reversion tothe initial state, as in the case of the Franz-Keldysh effect.

Electrochromic Material

The materials which form the electrochromic materials of the device ingeneral are electrical insulators or semiconductors. Thus are excludedthose metals, metal alloys, and other metal-containing compounds whichare relatively good electrical conductors.

The persistent electrochromic materials are further characterized asinorganic substances which are solid under the conditions of use,whether as pure elements, alloys, or chemical compounds, containing atleast one element of variable oxidation state, that is, at least oneelement of the Periodic System which can exist in more than oneoxidation state in addition to zero. The term "oxidation state" asemployed herein is defined in "Inorganic Chemistry," T. Moeller, JohnWiley & Sons, Inc., New York, 1952.

These include materials containing a transition metal element (includingLanthanide and Actinide series elements), and materials containingnon-alkali metal elements such as copper. Preferred materials of thisclass are films of transition metal compounds in which the transitionmetal may exist in any oxidation state from +2 to +8. Examples of theseare: transition metal oxides, transition metal oxysulfides, transitionmethalides, selenides, tellurides, chromates, molybdates, tungstates,vanadates, niobates, tantalates, titanates, stannates, and the like.Particularly preferred are films of metal stannates, oxides and sulfidesof the metals of Group (IV)B, (V)B and (VI)B of the Period System, andLanthanide series metal oxides and sulfides. Examples of such are copperstannate, tungsten oxide, cerium oxide, cobalt tungstate, metalmolybdates, metal titanates, metal niobates, and the like.

Additional examples of such compounds are the following oxides: MOoxides, e.g. MnO, NiO, CoO, etc.; M₂ O₃ oxides, e.g., Cr₂ O₃, Fe₂ O₃, Y₂O₃, Yb₂ O₃, V₂ O₃, Ti₂ O₃, Mn₂ O₃, etc.; MO₂ oxides, e.g., TiO₂, MnO₂,ThO₂, etc.; M₃ O₄ oxides, e.g., Co₃ O₄, Mn₃ O₄, Fe₃ O₄, etc.; MO₃oxides, e.g., CrO₃, UO₃, etc.; M₂ O₅ oxides, e.g., V₂ O₅ etc., Nb₂ O₅,Ta₂ O₅ etc.; M₄ O₆ oxides; M₂ O₇ oxides such as M₂ O₇ ; complex oxidessuch as those of the formula XYO₂ (X and Y being different metals),e.g., LiNiO₂, etc.; XYO₃ oxides, e.g., LiMnO₃, FeTiO₃, MnTiO₃, CoTiO₃,NiTiO₃, LiNbO₃, LiTaO₃, NaWO₃, etc.; XYO₄ oxides, e.g., MgWO₄, CdWO₄,NiWO₄, etc.; XY₂ O₆, e.g., CaNb₂ O₆ ("Niobite" oxides); X₂ Y₂ O₆, e.g.,Na₂ Nb₂ O₆ : Spinel structure oxides, i.e., of the formula X₂ YO₄, e.g.,Na₂ MoO₄, NaWO₄, Ag₂ MoO₄, Cu₂ MoO₄, Li₂ MoO₄, Li₂ WO₄, Sr₂ TiO₄, Ca₂MnO₄, etc.; XY₂ O₄, e.g., FeCr₂ O₄, TiZn₂ O₄, etc.; X₂ YO₅ oxides, e.g.,Fe₂ TiO₅, Al₂ TiO₅, etc.; and X₃ Y₃ O (ternary) oxides, e.g., Mo₃ Fe₃ O,W₃ Fe₃ O, X₃ Ti₃ O (where X is Mn, Fe, Co, etc.). For a discussion ofsome complex oxides, see Advanced Inorganic Chemistry, Cotten andWilkinson, p. 51, (1966), Interscience Publishers, Inc., New York andProgress in Inorganic Chem., Vol. 1, 465 (1959) Interscience Publishers,Inc., New York. Also included are nitrides, and the sulfidescorresponding to the above oxides. Hydrates of certain metal oxides mayalso be used, e.g., WO₃.H₂ O, WO₃.2H₂ O, MoO₃.H₂ O and MoO₃.2H₂ O.

A particularly advantageous aspect in the present invention is the useof two separate layers of identical electrochromic materials one layerbeing employed in the counterelectrode for the other layer. A preferredembodiment consists of tungsten oxide as the electrochromic colorelectrode and tungsten oxide and graphite as the counter-electrode.

While the general mechanism of persistent electrochromism is unknown,the coloration is observed to occur at the negatively chargedelectrochromic layer. Generally, the phenomenon of persistentelectrochromism is believed to involve cation transport such as hydrogenor lithium ions to the negative electrode where color centers form inthe electrochromic image layer as a result of charge compensatingelectron flow.

When the persistent electrochromic materials are employed as films,thickness desirably will be in the range of from about 0.1-100 microns.However, since a small potential will provide an enormous field strengthacross very thin films the latter, i.e., 0.1-10 microns, are preferredover thicker ones. Optimum thickness will also be determined by thenature of the particular compound being laid down as a film and by thefilm-forming method since the particular compound and film-formingmethod may place physical (e.g., non-uniform film surface) and economiclimitations on manufacture of the devices.

The films may be laid down on any substrate which, relative to the film,is electrically conducting. The electrically conductive material may becoated on another suitable substrate material including glass, wood,paper, plastics, plaster, and the like, including transparent,translucent, opaque or other optical quality materials. A preferredembodiment in the instant device would employ at least one transparentelectrode.

When tungsten oxide is employed as the electrochromic imaging materialand an electric field is applied between the electrodes, a bluecoloration of the previously transparent electrochromic layer occurs,i.e., the presistent electrochromic layer becomes absorptive ofelectromagnetic radiation over a band initially encompassing the red endof the visible spectrum, thereby rendering the imaging layer blue inappearance. Prior to the application of the electric field, theelectrochromic imaging layer was essentially non-absorbent and thustransparent.

Counter Electrode

As previously indicated, the counter-electrode may be any electricallyconductive material. Particularly advantageous is a layer ofelectrochromic material, as described previously. It is alsoadvantageous to use the same electrochromic material for the imagingarea and counter-electrode. A mixture of graphite and an electrochromicmaterial, or graphite alone may be used as the counter-electrode. Othermetallic counter-electrodes are disclosed in copending application, Ser.No. 41,154.

The invention may be further understood by reference to the drawings inwhich

FIG. 1 is a cross section of the electrochromic display device,

FIG. 2 is a front view of a single digital segment in an electrochromicdigital display,

FIG. 3 is a cross sectional view of the segment of FIG. 2, taken alongthe lines A--A,

FIG. 4 is a front view of a linear digital display according to theinvention.

As shown in FIG. 1, a conventional EC information display havingtransparent EC electrode 1, light colored, pigmented ion conductingmedium layer 2 and opaque counter-electrode 3. Layer 2 may be a porousself-supporting layer incorporating a pigment, or other desiredmaterials, and soaked in an electrolyte, e.g. sulfuric acid, or the likeas disclosed in commonly assigned application Ser. No. 41,154, filed May25, 1970. The EC electrode 1 forms the viewing surface and has atransparent or translucent substrate 5, e.g. glass, with a conductivelayer 6, e.g. tin oxide, and an electrochromic layer 7. Thecounter-electrode 3 is also a composite of a conductive layer 8 on asubstrate 9, and a counter-electrode material 10 such as carbon,tungsten oxide, or a mixture thereof. A suitable substrate for theviewing area and counter-electrode is NESA glass, which is glass havinga thin transparent layer of tin oxide.

When battery 11 is connected to make counter-electrode 3 negative, ECelectrode layer 7 colors. When the connections are reversed, EC layer 7erases (or bleaches).

In FIGS. 2, 3 and 4 are shown electrochromic devices with the EC layer 7in the form of a plurality of segments which may be selectivelyactivated to show numbers. FIG. 2 shows the number 4.

The invention should be usefully applied in EC displays for watches,clocks, calculators, telephone displays, automotive dashboards,instrument indicators and advertising displays.

EXAMPLE 1 Method For Preparing A Counter-Electrode

A counter-electrode was prepared as follows: Dixon Crucible Co.Graphokote No. 120 was brushed on a clean substrate on NESA glass. Whilethe Graphokote 120 film was still wet, WO₃ powder was sprinkled onto thesurface. Air drying for 1/2 hour at 25° C. and baking at 300° C. for 1/2hour followed. The WO₃ particles became embedded in the graphite film asthe electrode was air dried at 25° C. The electrode was cooled to 25° C.and soaked in a solution of glycerin-sulfuric acid 10:1 by volume for 24hours minimum, rinsed with acetone and baked at 90° C. for 1/2 hour todry. The resulting deposit was composed of approximately 0.5 gm/cm² WO₃on 2.0 mg./cm² Graphokote 120.

EXAMPLE 2 Method For Preparing Pigmented Spacing Ion-Conducting Layers

Type I pigment layers employ adhesive binders to hold the pigment powderin a self-supporting, porous film. Examples of Type I are: (a) Mix theratio 1 gm of Sun Yellow C pigment to 1 cc part A and 1 cc part BPeterson Clear Epoxy with 1 cc Peterson Epoxy Thinner. The mix issprayed onto Teflon sheet, cured for 1 hour at 65° C. and stripped fromthe Teflon film. The pigmented film. The pigmented film can be cut tosize for insertion into the EC device. (b) Beginning with Rohm and HaasLatex AC-34, mix 1 part to 9 parts (by volume) water. Add 1.5 cc of thismix to 1 gram of Sun Yellow C pigment and brush onto a microporouspolypropylene film, Celanese Plastics Company Celgard No. 2400 W. Roomtemperature dry for 1/2 hour then bake at 60° C. for 15 minutes. Cut thepigment on polypropylene film to size and assemble into the device.

Type II pigment layers feature a non-adhesive method of bonding of thepigment particles into a self-supporting layer. In one example 20 gramsof BaSO₄ powder was mixed with a dispersion containing 2 grams solids ofDupont TFE Dispersion 30B and water. The loose mix was blended in aWaring Blender for several minutes and heated in an oven at 120° C. todrive off the water. Ten grams of glycerin was added and stirred intothe mix. The mix was placed between sheets of Teflon film and squeezedto a thin film in a power roll. This rolling operation fibrillates theTeflon 30B and traps the pigment in the structure. The rolled film wasstripped from the Teflon sheets and the glycerin was extracted in anovernight water wash leaving a porous pigment film. The film was overdried and cut to size for assembly into an EC display.

Type III pigment layers are prepared by the paper-making process. In oneexample 3 grams of BaSO₄ powder, 3 grams of Sun Yellow C pigment and 1gram (solids) of acrylic fiber pulp were mixed with 300 cc of water andblended for 15 seconds. One cc of Cyanamid M560C flocculant was handstirred into the mix until the water cleared. The sheet was formed usinga common 6 inch diameter, paper-making machine. The sheet was roll sizedand dried on a rotating drum drier for 1 minute at 120° C. A sheet 0.015inch thick resulted which could be cut and assembled into an EC display.

EXAMPLE 3

An electrochromic device was constructed from two NESA glass plates. Oneconductive NESA plate was coated with a 0.5 micron thick evaporated filmof tungsten oxide. The other NESA plate was a counter-electrode as inExample 1. The glass plate so formed were pressed together with theelectrochromic and graphite films facing each other but separated by aspacing layer as described in Example 2, the layer having been saturatedwith a 1:10 ratio of concentrated sulfuric acid and glycerin. Thisdevice was cycled from color to clear and back to color at an appliedpotential of 1.1 volts D.C. with half cycles of 100 milliseconds. Thedevice underwent 5,000,000 cycles of switching at 60 cycles per minutewithout observable deterioration.

Previous methods attempting to correct the pigment separation problemresulted in slowing the switching speed. This invention eliminates theproblem without slowing the switching speed of the display. Theinvention also makes possible a variety of cosmetic effects notpreviously possible, allowing for improvements in appearance of thesebackground films. For example, the good insulating properties of thesefilms may permit the addition of reflecting metal particles to add"sparkle" to the display background.

The invention is expected to be useful in applications which includeinformation displays, indicators and others where the display is used inthe reflective mode. It is particularly useful in applications involvinglarge area display or applications in which shock and vibration ispresent, such as in automobile dashboard displays, and the like.

I claim:
 1. A varaible light transmission device which comprises lighttransmitting substrate having a persistent electrochromic material as alight modulating material, a counter-electrode, and a.[.self-supporting.]. .Iadd.porous separator .Iaddend.layer comprising apigment and an ion-conducting material .Iadd.in a binding medium.Iaddend.in contact with said .Iadd.light modulating .Iaddend.materialand counter-electrode.
 2. A variable light transmission device as inclaim 1, which comprises two layers, one with said electrochromicmaterial, and the other of said counter-electrode separated by said.[.self-supporting.]. .Iadd.porous separator .Iaddend.layer, disposedbetween a pair of conductive electrodes.
 3. The device of claim 2,wherein said counter-electrode is the type of the persistentelectrochromic material.
 4. The device of claim 3, wherein theelectrochromic materials in each said layer are identical.
 5. The deviceof claim 2, wherein at least one of the electrodes is substantiallytransparent.
 6. The device of claim 4, wherein said electrochromicmaterials are WO₃.
 7. A device of claim 1, wherein said.[.self-supporting.]. .Iadd.porous separator .Iaddend.layer contains acolor pigment.
 8. The device of claim 1, wherein said.[.self-supporting.]. .Iadd.porous separator .Iaddend.layer is a poroussheet saturated with an electrolyte. .Iadd.
 9. The device of claim 1wherein said porous separator layer is self-supporting. .Iaddend..Iadd.10. The device of claim 9 wherein the self-supporting porous layer is aseparate pre-cut element. .Iaddend. .Iadd.
 11. In an electrochromicdisplay of the type having a first substrate with selectively actuatabletransparent electrodes and first electrochromic layers thereon, and alsohaving a second substrate with counter-electrode and secondelectrochromic layer thereon, the improvement comprising:porousseparator means applied as a layer and uniformly and closely spacingsaid first and second substrate members, the pores of said separatormeans being filled with liquid electrolyte, and pigment means held bysaid separator means and selected to provide contrast with said firstelectrochromic layers and disposed to hide said second electrochromiclayer. .Iaddend..Iadd.
 12. The combination according to claim 11,wherein said porous separator means comprises porous polypropylene..Iaddend..Iadd.
 13. The combination according to claim 11, wherein saidpigment means is premixed in said separator means. .Iaddend.